A detailed study of cardiovascular status will be carried out in a unique group of patients who have previously sustained sudden unexpected ventricular fibrillation, and are at high risk for recurrent serious arrhythmias or sudden death. The goal is a determination of the most significant risk factors in this population. The studies will determine: (1) acute and chronic electrophysiologic status of the patients; (2) hemodynamic status; (3) coronary artery anatomy; (4) the presence of recent or old myocardial infarction(s) and chronic myocardial ischemia; (5) influence of therapy on prodromal rhythm disturbances and long-term survival. Patient input will derive from a group of successful cardiac resuscitations by the City of Miami fire rescue squad, and the cardiopulmonary arrest team of Jackson Memorial Hospital. Studies of individual patients will be divided into 4 phases: Phase I studies - electro-cardiography at the scene of the cardiopulmonary arrest, and initial ECG and enzyme studies in the coronary care units; Phase 2 - computerized rhythm trend studies, and acute hemodynamic and intracardiac electrophysiologic studies, in the coronary care unit; plus radionuclide imaging studies; Phase 3 - cardiac catheterization and coronary angiography, complete intracardiac electrophysiologic studies, and intraoperative and postoperative epicardial electrogram studies; and Phase 4 - long-term electrophysiologic studies by three different methods, plus followup hemodynamic, angiographic, and natural history studies. Based on previous experience, 300 patients are expected to enter Phase I in a 5 year period, 250 of whom will complete all or part of Phase 2, 175 of whom will complete all or part of Phase 3, and 150 of whom will enter the long-term Phase 4 studies. The attrition in Phase 4 is estimated to be such that 75 patients will remain at the end of 5 years. The characteristics recorded in survivors and nonsurvivors of each phase of the study will be compared. The information obtained could eventually lead to an ability to apply controls to identified risk factors in an attempt to prevent the initial cardiac arrest, thus decreasing the large prehospital mortality.